1. Field of the Invention
The present invention relates to an optical position measuring system, which provides scanning signals free of harmonic waves and includes a scale with a scale graduation, a scanning unit with a light source, at least one periodic scanning graduation and a detector arrangement.
2. Discussion of Related Art
In connection with known incremental position measuring arrangements on an optical basis, a scale with a periodic scale graduation is customarily scanned by a periodic scanning graduation. The latter is arranged, together with further components, for example, a light source and a detector arrangement, in a scanning unit, which is movable in relation to the scale in at least one measuring direction. In the ideal case an exactly sinusoidal scanning signal exists, which can be further processed, respectively further subdivided, in suitable electronic follow-up devices by known interpolation methods. However, the actually resultant shape of the detected scanning signals customarily differs more or less greatly from the ideal sinusoidal shape. Various causes can be responsible for this, for example deviations from ideal conditions of the graduation periods on the scale side and/or scanning side, fuzzy edges of the graduation mark, etc. Therefore, in actual use the resultant scanning signals contain harmonic waves, which in particular can hamper further signal processing in the form of interpolation. It is therefore basically attempted to minimize the harmonic wave content of the scanning signals as much as possible.
A multitude of attempts for filtering harmonic waves are already known from the prior art. For example, in the present case reference is made to steps for filtering harmonic waves known from DE 195 08 700 C1 or U.S. Pat. No. 5,068,530. For eliminating the particularly interfering third and fifth harmonic wave, it is proposed in these publications to arrange the graduation marks on the scanning plate at defined offset spacings from the respective nominal positions of the graduation marks. The required offset spacings always result from the respectively to be eliminated harmonic wave.
If, based on this filtering principle, it is now intended to eliminate, for example two dominant harmonic waves, for example the third and fifth harmonic waves, it is proposed in accordance with FIG. 4 of DE 195 08 700 C1 to provide the required offset spacings on the scanning plate between the individual graduation marks, as well as between groups of graduation marks. As a result, two defined offset spacings of the graduation marks from the original nominal positions therefore exist on the scanning plate in the measuring direction.
However, this filtering principle can no longer be employed in connection with certain incremental scanning operations, in particular in case Vernier scanning is intended. This is understood to be a scanning principle wherein the scale graduation and the scanning graduation have slightly different graduation periods, so that in accordance with the interaction of the light beams emitted by the light source a spatially periodic Vernier fringe pattern of a defined fringe pattern period P results in a detection plane by the two graduations. In this case the fringe pattern period P is as a rule clearly greater than the graduation periods TPM, TPA of the scale and the scanning graduations. If now a filtering principle in accordance with FIG. 4 of the mentioned DE 195 08 700 C1 were employed for eliminating the third and fifth harmonic waves, this would result in an interference with the fixed phase relationships between the scale and the scanning grating which are required for scanning. The creation of a Vernier fringe pattern of a correct phase would no longer be possible.
This problem could be sidestepped in principle with the use of a filter variation as disclosed, for example, in FIG. 4 of the mentioned U.S. Pat. No. 5,068,530, the entire contents of which is incorporated herein by reference. It is provided there to divide the scanning graduation on the scanning plate into several partial tracks perpendicularly with respect to the measuring direction, i.e. in the longitudinal direction of the graduation marks. Accordingly, with this variation the required offset spacings for filtering would be provided in the longitudinal direction of the graduation marks and not, as mentioned above, in the measuring direction. Vernier scanning would be possible in principle with such filtering. However, the sensitivity of the scanning, or of the harmonic wave filtering, in regard to twisting of the scale and the scanning unit around an axis perpendicularly in relation to the scale possibly resulting during measuring operations or in the course of assembly, is problematic here. In what follows, such twisting will also be called Moiré twisting.